1. FIELD OF THE INVENTION:
This invention relates to a method for producing N-(4-fluorophenyl)-2,3-dichloromaleimide which has excellent agricultural fungicidal activity.
2. DESCRIPTION OF THE PRIOR ART:
It has been reported that N-(4-fluorophenyl)-2,3-dichloromaleimide as expressed by the formula ##STR1## exhibits excellent activity as an agricultural fungicide and is effective for protecting plants from various diseases such as anthracnose rot of cucumbers (Collectotrichum lagenarium), late blight of tomatoes (Phytophthora infestans), citrus melanose (diaporthe citri), citrus scab (elsinoe fawcetti) and red dust disease of coffees (Japanese Pat. Nos. 712,681 and 671,884, U.S. Pat. No. 3,734,927, British Pat. No. 1,324,910, etc.)
From these patents, it is already known that N-(4-fluorophenyl)-2,3-dichloromaleimide can be produced either by reacting 2,3-dichloromaleic acid, an anhydride or an ester thereof with 4-fluoroaniline in an organic solvent or by the intra-molecular dehydration of N-(4-fluorophenyl)-2,3-dichloromaleamic acid in an organic solvent. Japanese Laid-open Patent Application Nos. Sho 49(1974)-110,661 and Sho 49(1974)-110,662 report that the reactions can be advantageously effected in the presence of a strong acid such as p-toluene-sulfonic acid which serves as a catalyst.
However, N-(4-fluorophenyl)-2,3-dichloromaleimide obtained by the above-mentioned conventional methods has a comparatively large grain size and therefore, must be pulverized two or more times for use as an agricultural fungicide. Moreover, the conventional reaction methods employ an organic solvent and accordingly require an additional operation for recovering the solvent and encounter the problem of corrosion of the apparatus by the reaction mixture. For example, when acetic acid is used as the organic solvent, an excessive amount of dichloromaleic acid, or one of the starting materials, dissolves in the acetic acid to form a solution which has extremely high corrosive action and attacks the centrifugal filter and the distillation apparatus employed for recovering the solvent. To eliminate this danger, expensive materials such as Monel metal or Hastelloy alloy C must be employed. When an aromatic hydrocarbon such as benzene is used as the organic solvent, the reaction cannot fully proceed unless a dehydrating agent or catalyst such as p-toluene-sulfonic acid is employed.